Studies on rearrangements in derivatives of grandiflorenic acid. Part 1. Reaction of the epoxides of methyl (–)-kaur-9(11)-en-19-oate and (–)-kaur-9(11)-en-19-oic acid with boron trifluoride-diethyl ether either in the absence or in the presence of N-nitrosomethylurea. Formation of two diterpenes of a new skeletal type

Abstract

On treatment with boron trifluoride-diethyl ether in benzene the epoxide (3b) of methyl (–)-kaur-9(11)-en-19-oate (2c) yielded an aldehyde (5a). During the epoxidation of compound (2c) with m-chloroperbenzoic acid it was observed that in the presence of N-nitrosomethylurea the expected epoxide (3b) was not obtained, but instead compound (4) was isolated. Detailed studies showed that epoxide (3b) is an intermediate for the formation of compound (4) from compound (2c), and aldehyde (5a) is also involved in its mechanistic pathway. Epoxidation of aldehyde (5a), followed by acid treatment, led to compound (4). On rupture with boron trifluoride–diethyl ether in acetic anhydride epoxide (3b) afforded acylal (5b) in good yield.(–)-Kaur-9(11)-en-19-oic acid (2b) behaved similarly. On rupture with boron trifluoride-diethyl ether its epoxide (3a) gave also an aldehyde, (5c), while on treatment with boron trifluoride–diethyl ether in acetic anhydride it yielded acylal (5d).